Swelling behind the knee is most commonly caused by a Baker’s cyst, a fluid-filled sac that forms when lubricating fluid from the knee joint builds up in the hollow space at the back of your knee. Other causes include blood clots, arthritis-related inflammation, and less commonly, vascular problems. The location and feel of the swelling, along with any accompanying symptoms, help narrow down what’s going on.
Baker’s Cyst: The Most Common Cause
Your knee joint produces a slippery fluid that reduces friction when you bend and straighten your leg. When something irritates the joint, it can overproduce this fluid. The excess fluid migrates to the back of the knee and fills a small cushioning sac called a bursa, creating a visible, sometimes golf-ball-sized bulge. That bulge is a Baker’s cyst.
A Baker’s cyst almost always points to an underlying knee problem. The most frequent triggers are osteoarthritis, rheumatoid arthritis, and cartilage tears. In a study of 60 people with rheumatoid arthritis, nearly half had a Baker’s cyst, and some had cysts behind both knees. Among healthy people of the same age, cysts were rare. That gives you a sense of how strongly joint inflammation drives this kind of swelling.
A small Baker’s cyst may cause no symptoms at all. Larger ones typically produce a feeling of tightness or pressure behind the knee that worsens when you fully bend or extend your leg. Some people notice the swelling gets bigger after prolonged standing or activity. In rare cases, a Baker’s cyst can rupture, sending fluid down into your calf. A ruptured cyst causes sudden sharp pain behind the knee, redness, and calf swelling that can look a lot like a blood clot.
Deep Vein Thrombosis
A blood clot in a deep leg vein (DVT) can cause swelling, warmth, and pain behind the knee or in the calf. The swelling from a DVT tends to feel more diffuse than the distinct lump of a Baker’s cyst. Your skin may look reddish or feel noticeably warmer on the affected side. DVT is more likely after long periods of immobility, such as a long flight, bed rest after surgery, or a leg injury. It’s also more common if you take hormonal birth control, are pregnant, or have a clotting disorder.
DVT is the main reason posterior knee swelling should not be ignored. If a clot breaks loose and travels to your lungs, it becomes a pulmonary embolism, which is a medical emergency. Because a ruptured Baker’s cyst and a DVT can look nearly identical from the outside, imaging is often needed to tell them apart.
Arthritis-Related Swelling
Both osteoarthritis and rheumatoid arthritis cause inflammation inside the knee joint, and that inflammation doesn’t always stay in the front. Fluid and swelling can wrap around to the back of the knee even without forming a distinct cyst. With osteoarthritis, you’ll usually notice stiffness that’s worst in the morning or after sitting, and the swelling tends to come and go with activity. With rheumatoid arthritis, the swelling is more persistent, often affects both knees, and may be accompanied by warmth, fatigue, and swelling in smaller joints like your hands or wrists.
If you already have a diagnosis of arthritis and notice new swelling behind the knee, it likely means your joint inflammation isn’t well controlled. Treating the underlying arthritis, rather than the swelling itself, is what resolves the problem long term.
Popliteal Artery Aneurysm
Far less common but worth knowing about: the main artery behind your knee can develop a weak, ballooned-out section called an aneurysm. Most popliteal aneurysms cause no symptoms and are discovered incidentally during imaging for something else. When they do cause problems, it’s usually because a clot forms inside the aneurysm. That clot can block blood flow to your lower leg, causing pain, numbness, coldness, and muscle weakness below the knee.
A ruptured popliteal aneurysm causes swelling, bruising, and pain behind the knee, along with difficulty moving the affected leg. This is a medical emergency requiring immediate evaluation. Popliteal aneurysms are most common in men over 65, especially those with a history of aneurysms elsewhere in the body.
How the Cause Is Identified
A physical exam can often identify a Baker’s cyst based on the location, shape, and feel of the swelling. Your doctor will check whether the lump is soft and fluid-filled or firm and pulsating, which helps distinguish a cyst from a vascular problem. They’ll also look for signs of a blood clot, such as asymmetric calf swelling, warmth, and tenderness along the vein.
When the diagnosis isn’t clear from the exam alone, imaging fills in the gaps. Ultrasound is typically the first step because it’s fast, widely available, and excellent at differentiating a fluid-filled cyst from a blood clot or aneurysm. An MRI provides more detail about the structures inside the knee, including cartilage tears or ligament damage that may be driving excess fluid production. X-rays can reveal arthritis-related bone changes but won’t show soft tissue swelling well on their own.
What Treatment Looks Like
Treatment depends entirely on the cause. A Baker’s cyst that isn’t causing much discomfort often needs no treatment at all. Many resolve on their own once the underlying joint issue is addressed. For a cyst that’s painful or limiting your movement, draining the fluid with a needle provides quick relief, though the cyst can refill if the root cause persists. Treating the knee condition generating the extra fluid, whether that’s managing arthritis or repairing a cartilage tear, is what prevents the cyst from coming back.
For arthritis-driven swelling, reducing joint inflammation through physical therapy, anti-inflammatory medications, or disease-modifying treatments (in the case of rheumatoid arthritis) shrinks the swelling over time. Icing the back of the knee and elevating your leg can help manage flare-ups in the short term.
DVT requires prompt treatment with blood thinners to prevent the clot from growing or traveling. A popliteal aneurysm that’s causing symptoms or has reached a certain size typically requires a procedure to repair or bypass the weakened artery section.
Swelling in One Knee vs. Both
If only one knee is swollen, a Baker’s cyst, cartilage injury, or DVT are the most likely explanations. These conditions tend to affect one side at a time. If both knees are swollen behind, a systemic condition like rheumatoid arthritis or gout is more probable, since these diseases affect joints symmetrically. Bilateral swelling can also result from general fluid retention related to heart, kidney, or liver issues, though in those cases you’d usually notice swelling in your ankles and feet as well.